Because, no matter what culture you're part on, having your kill killed is going to make you be against the current rulers.
No surprise small Spanish groups of sailors and great armies of native allies could topple several empires through America.
The Spaniards allied themselves early with these would be victims, who therefore helped remove the numeric superiority the Aztecs would have otherwise enjoyed.
These allied tribes (see the Tlaxcala) then became a privilege class among the natives.
The guns, and the horses, obviously helped - but we're talking 4 orders of magnitude advantage if the Spaniards had not allied themselves with the locals. Even pandemics cannot erase that advantage (and they take time to spread)
I don't think so. It's probably mostly down to what Jared Diamond describes - Guns, Germs, and Steel.
The occasional score of children and conquests slaughtered voluntarily is dwarfed by the numbers of natives that died from smallpox (f.e.).